The automobile society of today has many serious problems such as traffic congestion. To solve such problems, ITS (Intelligent Transport System) has been proposed, and specific systems such as VICS (Vehicle Information Communications System) and ETC (Electronic Toll Collection System) have been put into practice.
Furthermore, in recent years, as a system relevant to ITS, an inter-vehicle communication system is attracting attention for the purpose achieving a safe automobile society. An inter-vehicle communication system is used for performing wireless communication between vehicles. An inter-vehicle communication system is different from a mobile wireless communication system in which a base station implements centralized control on mobile terminals such as mobile telephones. Specifically, in an inter-vehicle communication system, decentralized autonomous control is implemented so that the respective vehicles perform communication with each other in accordance with a certain rule. That is to say, ad hoc communication is performed. Generally, the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method is known as an example of a decentralized autonomous control system, and the CSMA/CA method is proposed as an inter-vehicle communication system.
In the CSMA/CA method, it is confirmed whether a communication path including a carrier frequency has not been continuously used for more than a certain time period, with the use of a received power level (carrier sense). When the confirmation result indicates that the communication path has not been used, a vehicle uses the communication path to transmit data. At the stage of carrier sensing, when it is confirmed that the communication path is used, i.e., the received power level exceeds the carrier sense level, waiting is employed until usage of the communication path ends. Subsequently, when it is confirmed that usage of the communication path has ended, a vehicle starts transmission after a random waiting time passes. Transmission is started after a random waiting time passes because if a vehicle starts transmission immediately after usage of the communication path ends, there is a high possibility that the transmission collides with that from another vehicle.
In the above method, when the number of vehicles attempting to perform communication is less than or equal to a certain number, the receiving throughput increases, and inter-vehicle communication is performed with good performance. However, when the number of vehicles attempting to perform communication exceeds the certain number, the following hidden terminal problem arises.
The hidden terminal problem arises as follows. Two vehicles are located outside each other's communication range, and are thus located outside the target of carrier sensing. These two vehicles transmit data at the same time with the use of the same communication path. A receiving vehicle is located at a position where communication can be performed with the two transmitting vehicles. In this situation, the data signals from the two transmitting vehicles collide with each other. This problem is referred to as a hidden terminal problem. As a measure to counter this hidden terminal problem, an RTS/CTS (Request to Send/Clear to Send) method may be used. However, the RTS/CTS method is effective when a large amount of data is transmitted. The RTS/CTS method is ineffective when transmitting a relatively small amount of data as in inter-vehicle communication.
Japanese Laid-Open Patent Publication No. 2004-062381 describes an inter-vehicle communication device that smoothly performs inter-vehicle communication at intersections. The inter-vehicle communication device acquires the present position information of the vehicle in which the device is installed (self vehicle), and also selects a communication channel used for transmitting wireless signals from the present position of the self vehicle with the use of map information including the association between positions, roads, and communication channels. Specifically, it is described that communication channels are assigned to roads connected to intersections, so that inter-vehicle communication is smoothly performed at intersections (see paragraph (0015) of Japanese Laid-Open Patent Publication No. 2004-062381).
Furthermore, Japanese Laid-Open Patent Publication No. 2006-254215 describes an inter-vehicle communication device for supporting a vehicle to pass through a road where it is difficult for two vehicles to pass by each other. The inter-vehicle communication device assigns different diffusion codes for transmission to the respective roads, and transmits information to other vehicles with the use of these diffusion codes.
However, with the above-described devices, communication channels and diffusion codes are assigned to the respective roads. Therefore, the above devices are not capable of solving a hidden terminal problem arising in the same road.